Bitumen is used in paving, roofing, joint compound adhesives and other applications. Bitumen is an inexpensive material for these uses, but has many shortcomings, including low flexibility at low service temperatures and low modulus at high service temperatures. These properties as well as rut resistance and cold flow would be improved if service temperature viscosities could be increased for compositions with similar processing temperature viscosities. Some of these deficiencies in physical properties may be at least partially overcome by including in the bitumen composition various elastomeric polymers such as styrenic copolymers.
Many asphalts are not compatible with styrenic copolymers such as styrene-butadiene-styrene (SBS) copolymers, hydrogenated versions thereof (SEBS) or styrene-butadiene rubbers (SBR). Such incompatible blends exhibit phase instability which manifests itself in bulk phase separation such as may occur in bulk storage at relatively high temperatures. Upon standing for long periods of time, a polymer rich (asphaltene lean) phase forms in the upper layers and a polymer lean (asphaltene rich) phase forms at the bottom. Generally, in such asphalts one also observes a coarse dispersion of the polymer and relatively poor physical properties Several problems then result. The dispersion requires constant agitation to maintain a relatively homogeneous mixture. Phase separation may also occur with time under service conditions encountered in field applications such as road paving, roofing or other industrial applications Also, in such cases, more polymer is often required to effect the desired improvements in physical properties of the polymer/bitumen blends.
Various approaches have been tried to increase the compatibility of these blends. For example, U.K. patent specification 1,143,895 teaches adding carbon black to such blends to get increased properties such as elongation at break, tensile stress at 300% elongation and tensile stress at break. However, the compatibility of these compositions, as measured by the amount of phase separation during hot storage of the composition, is deficient because such blends are still thermodynamically unstable. Also, polystyrene has been added to these blends, as described in U.S. Pat. No. 3,978,014, to increase their compatibility. Only certain bitumens may be utilized in such compositions due to the inherent incompatibility between bitumens and block copolymers. In order to achieve compatibility a significant amount of polystyrene must be present and this increases the cost of the bitumen blend without improving ductility or flexibility.
It is therefore an objective of the present invention to provide compositions of bitumen and block copolymers which are compatible and thus exhibit hot storage stability and improved viscoelastic properties. Excellent viscoelastic properties include lower viscosities at similar processing temperatures and equivalent or higher viscosity and elasticity at high service temperatures.